thanks all. I appear to have fixed it.

what I did was:
Start, Control Panel, Network Connections, right click on the "wireless network connection" in the list, properties, wireless networks tab, click on the wireless network under the "preferred networks" area, click Remove.

Click Add - ALL it asked for was the SSID of the network and I had to put in the network key. I didn't touch any of the other tabs. In particular, previously I noticed that on the "authentication" tab the top item "IEEE 802.1x" had been ticked; it now wasn't and I left it unticked.

I hadn't tried to delete the network before as I was "scared it would ask me for things I didn't know or ask me to make a decision between two things I didn't understand or ask me for a CD I had to find". But it didn't do any of that!

That was it. It connected and now stays connected.

Thanks for your help guys!!

I hope somebody else will stumble across this thread and be helped by your suggestions